Oxygen Hydration Mechanism for the Oxygen Reduction Reaction at Pt and Pd Fuel Cell Catalysts

We report the reaction pathways and barriers for the oxygen reduction reaction (ORR) on platinum, both for gas phase and in solution, based on quantum mechanics calculations (PBE-DFT) on semi-infinite slabs. We find a new mechanism in solution: O₂ → 2O_ad (<i>E</i>_act = 0.00 eV), O_ad + H₂O_ad → 2OH_ad (<i>E</i>_act = 0.50 eV), OH_ad + H_ad → H₂O_ad (<i>E</i>_act = 0.24 eV), in which OH_ad is formed by the hydration of surface O_ad. For the gas phase (hydrophilic phase of Nafion), we find that the favored step for activation of the O₂ is H_ad + O_2ad → HOO_ad (<i>E</i>_act = 0.30 eV) → HO_ad + O_ad (<i>E</i>_act = 0.12 eV) followed by O_ad + H₂O_ad → 2OH_ad (<i>E</i>_act = 0.23 eV), OH_ad + H_ad → H₂O_ad (<i>E</i>_act = 0.14 eV). This suggests that to improve the efficiency of ORR catalysts, we should focus on decreasing the barrier for O_ad hydration while providing hydrophobic conditions for the OH and H₂O formation steps

Finding Correlations of the Oxygen Reduction Reaction Activity of Transition Metal Catalysts with Parameters Obtained from Quantum Mechanics

To facilitate a less empirical approach to developing improved catalysts, it is important to correlate catalytic performance to surrogate properties that can be measured or predicted accurately and quickly, allowing experimental synthesis and testing of catalysts to focus on the most promising cases. Particularly hopeful is correlating catalysis performance to the electronic density of states (DOS). Indeed, there has been success in using just the center of the d-electron density, which in some cases correlates linearly with oxygen atom chemisorption energy, leading to a volcano plot for catalytic performance versus “d-band center”. To test such concepts we calculated the barriers and binding energies for the various reactions and intermediates involved in the oxygen reduction reaction (ORR) for all 12 transition metals in groups 8–11 (Fe–Cu columns). Our results show that the oxygen binding energy can serve as a useful parameter in describing the catalytic activity for pure metals, but it does not necessarily correlate with the d-band center. In addition, we find that the d-band center depends substantially on the calculation method or the experimental setup, making it a much less reliable indicator for ORR activity than the oxygen binding energy. We further examine several surfaces of the same pure metals to evaluate how the d-band center and oxygen binding energy depend on the surface

Density Functional Theory Study of Pt₃M Alloy Surface Segregation with Adsorbed O/OH and Pt₃Os as Catalysts for Oxygen Reduction Reaction

Using quantum mechanics calculations, we have studied the segregation energy with adsorbed O and OH for 28 Pt₃M alloys, where M is a transition metal. The calculations found surface segregation to become energetically unfavorable for Pt₃Co and Pt₃Ni, as well as for the most other Pt binary alloys, in the presence of adsorbed O and OH. However, Pt₃Os and Pt₃Ir remain surface segregated and show the best energy preference among the alloys studied for both adsorbed species on the surface. Binding energies of various oxygen reduction reaction (ORR) intermediates on the Pt(111) and Pt₃Os­(111) surfaces were calculated and analyzed. Energy barriers for different ORR steps were computed for Pt and Pt₃Os catalysts, and the rate-determining steps (RDS) were identified. It turns out that the RDS barrier for the Pt₃Os alloy catalyst is lower than the corresponding barrier for pure Pt. This result allows us to predict a better ORR performance of Pt₃Os compared to that of pure Pt